1. Field of the Invention
This invention relates to a light emitting device and a method of fabricating the same.
2. Description of the Related Art
After years of advancement, materials and element structures applied to light emitting elements such as light emitting diode and semiconductor laser devices have become more and more closer to the theoretical limits in terms of photo-electric conversion efficiency inside the device. Efficiency of extracting light from the device is, therefore, a critical issue for every effort of obtaining the device of further higher luminance. For example, a light emitting device having the light emitting layer portion thereof composed of an AlGaInP alloy can be realized as a high-luminance device, by adopting a double heterostructure in which a thin AlGaInP (or GaInP) active layer is sandwiched between an n-type AlGaInP cladding layer and a p-type AlGaInP cladding layer, both having larger band gaps. This sort of AlGaInP double heterostructure can be formed by epitaxially growing the individual layers composed of AlGaInP alloy on a GaAs single crystal substrate, relying upon capability of lattice matching of AlGaInP to GaAs. When such product is used as a light emitting, it is often to use the GaAs single crystal substrate (simply referred to as “GaAs substrate” on occasions, hereinafter) as a device substrate without modification. The AlGaInP alloy composing the light emitting layer portion, however, has a band gap larger than that of GaAs, and is therefore disadvantageous in that the emitted light is absorbed by the device substrate portion, so that a sufficient level of light extraction efficiency is hard to obtain.
Japanese Laid-Open Patent Publication “Tokkai” No. 2001-339100 discloses a technique of separating the growth-assisting GaAs substrate, and bonding a reinforcing device substrate (electro-conductive one) onto the surface exposed by separation, while placing a reflective Au layer in between. Nikkei Electronics Oct. 21, 2002, p. 124-132 discloses a light emitting device raised in reflection intensity, by configuring a reflective layer using Al having a smaller wavelength dependence of reflectivity than that of Au. The device disclosed in the Nikkei Electronics Oct. 21, 2002, p. 124-132 is configured as having an Al reflective layer disposed between the light emitting layer portion and the device substrate composed of a silicon substrate, and further as having a Au layer interposed between the Al reflective layer and the silicon substrate, so as to facilitate bonding between the silicon substrate and the light emitting layer portion. More specifically, the Au layer is formed so as to cover the Al reflective layer formed on the light emitting layer portion side, on the other hand, the Au layer is formed also on the silicon substrate side, and these Au layers are brought into close contact with each other so as to complete the bonding.
Both of Japanese Laid-Open Patent Publication “Tokkai” No. 2001-339100 and Nikkei Electronics Oct. 21, 2002, p. 124-132 stand on a technical idea that, in view of improving the light emitting efficiency of the light emitting device, use of a light-absorbing GaAs substrate produces thousand evils and no good, and aims principally at completely removing the GaAs substrate. It should, however, be said too wasteful indeed to completely remove the GaAs substrate, which is considerably more expensive as compared with silicon substrates, without paying no attention about use thereof and to separately provide the reinforcing silicon substrate, yet the top priority is placed on light extraction efficiency. The GaAs substrate assisting growth of the light emitting layer portion also plays a role of providing necessary strength against handling in the device fabrication, so that it is obvious that, if the GaAs substrate is removed, only the very thin light emitting layer portion will never ensure necessary strength against handling or the like. According to the above-described literatures, the GaAs substrate is removed from the light emitting layer portion, the silicon substrate is therefore bonded to the light emitting layer portion while placing the Au layer in between, making use of this silicon substrate as a reinforcing substrate in place of the GaAs substrate, but such process needs a process step of bonding another new substrate.